Current methods to introduce chemical modifications into oligonucleotides have limitations. Many non-nucleosidic phosphoramidite reagents are limited to single modifications at the 5′ terminus, thus terminating chain elongation at the point of introduction. Those designed for multiple incorporations such as 1,2-diol backbone phosphoramidite reagents also suffer some drawbacks. The internucleotide distance, when incorporated internally, results in a consticted internucleotide phosphate distance one carbon atom shorter than the natural DNA structure. Further, the 1,2-diol backbone can participate in a dephosphorylation reaction due to a highly favorable 5-membered cyclic phosphate intermediate, resulting in cleavage of the label. Other reagents suffer from poor design in protecting label moieties. For example, some biotin phosphoramidite reagents do not protect its urea moiety. Hence, phosphoramidites can react at this active position of biotin in unwanted side reactions.
The 1,3 diol reagents of Nelson et at have proven to be superior, overcoming the above disadvantages, albeit, improved protection of label moieties have not been addressed. The subject of this invention builds upon the advantages of the 1,3 diol reagents by utilizing a serinol backbone. This backbone is versatile, readily available, and allows for convenient preparation of reagents. The purpose of this invention is to overcome the disadvantages encountered in the prior art by providing improved reagents to directly modify or label oligonucleotides via automated DNA synthesis.